1. Field of the Invention
This invention pertains generally, to contact systems for DC machines and, particularly, to a fluid contact system.
2. Description of the Prior Art
High-power turbo generators and Dc machines require heavycurrent contacts which, oftentimes, are required to handle large currents while the rotor turns at high speed. Sliding carbon brush contacts have been used in the past but these present difficulties due to the fact that they have a relatively low current-carrying capacity -- on the order of 50A/cm.sup.2. For large current machines, therefore, much more space would be required for the contact brushes and their holders. Further because of the limited contact area, brush contacts result in relatively large ohmic losses, accompanied by field distortion leading, generally, to higher losses.
Uniform current distribution with its resulting lower ohmic losses it provided by known fluid contacts. Generally, these fluid contacts comprise metallic contact elements with a highly conductive contact fluid interpose therebetween. The relatively large contact surface brings about improved current conduction and low ohmic losses with corresponding low contact voltages. These fluid contacts allow for conduction of larger currents and higher rotor speeds. In a typical arrangement a fluid contact is provided between the rotor and the stator which is filled with a contact fluid under pressure. The injection pressure of the contact fluid is controlled such that it cancels whatever centrifugal forces are generated by the rotating rotor, through frictional forces acting upon the contact fluid by the rotor. However, the liquid metal portion of the contact fluid tends to flow off in an axial direction, through a ring canal, where it is collected and pumped therefrom. A gas seal is provided on each axial end of the gap between the outer perimeter of the rotor and the stator. This seal tends to reduce the loss of contact fluid. However, at higher rotational speeds, where larger centrifugal forces occur, a gas pressure face assumes a relatively high number -- about 8.5 atmospheres for that situation where sodium-potassium is used as the liquid metal, where the rotor diameter is one meter, where the rotor speed is 3,000 r.p.m. and where the height of the ring of contact fluid is 2 cm. Thus, with the higher rotational speeds and the tendency of fluid to flow off under increasing pressure towards the sides, the increasing counter-pressure of the lateral gas seals supplied by a relatively complex controlled system, results in a certain mixing of the gas forming the seal with the contact fluid. As a result, the conductivity of the fluid is reduced. This problem leads to a requirement for the use of separators to remove the gas contaminating the contact fluid.
It is therefore a primary object of this invention to provide a simplified fluid contact system which retains the contact fluid between the rotor and stator, even under higher rotor rotational speeds. It is another object of the invention to provide a contact fluid which is either ferromagnetic itself, or which contains ferromagnetic particles suspended in a non-magnetic metallic liquid, which fluid reacts with a magnetic field such that the fluid is restrained in the gap.